On Wikipedia, it says https://en.wikipedia.org/wiki/Georgi–Glashow_model#Breaking_SU(5):
Fermions transforming as a 1 under SU(5) are now thought to be necessary because of the evidence for neutrino oscillations, unless a way is found to introduce a tiny Majorana coupling for the left-handed neutrinos.
This text tries to explain how the (1,2)−1/2 of left-handed lepton (with a upper component of the doublet as left-handed neutrino) and (1,1)0 as right-handed neutrino, can get a mass.
My question is that how many ways can (1,2)−1/2 and (1,1)0 can get a mass?
It seems to me
1). (1,1)0 alone can get a Majorana mass.
2). (1,2)−1/2 and (1,1)0 together can get a Dirac mass.
3). Is there any difficulty for (1,2)−1/2 alone (say a upper component of the doublet as left-handed neutrino) to get a Majorana mass? I do not see an immediate no go reason. But Wikipedia seems to say this is a big new discovery? See Wikipedia: "unless a way is found to introduce a tiny Majorana coupling for the left-handed neutrinos." Why?
The original text goes like:
This post imported from StackExchange Physics at 2020-12-01 17:41 (UTC), posted by SE-user annie marie heart